In a so-called percussion tap drilling process, a percussion tap drill bit is used both to create a core bore and to cut an inner thread in a joint tool bit stroke. The percussion tap drill bit has a primary cutting edge at its drill bit tip and a thread profile having at least one thread cutting tooth, this profile trailing in a tap drilling direction. In the method, first of all, the tap drill bit stroke occurs and, subsequently, a reverse stroke occurs in the opposite direction. In the tap drilling stroke, the primary cutting edge of the bit produces, on the one hand, the core-hole bore and, on the other hand, the thread profile of the tool bit produces the inner thread at the inner wall of the core-hole bore until a useable desired thread depth is reached. For this purpose, in the tap drilling stroke, the tap drill bit is operated during a tap drilling advance at a tap drilling rotational speed that is synchronized therewith. In the oppositely directed reverse stroke that follows, the tap drill bit is guided in a reverse direction out of the tapped bore and, in fact, this is done with an oppositely directed reverse feed as well as with a reverse rotational speed that is synchronized thereto. It is thereby ensured that the thread profile of the tap drill bit in the thread path of the inner thread is moved out of the tapped bore without any load.
In the above method, shavings are produced in the tap drilling stroke and are conveyed out from the tapped bore in a shavings discharge direction that is opposite to the tap drilling direction. In this case, the shavings moving in the shavings discharge direction collide with the thread flanks of the inner thread that face the shavings. Therefore, at the thread flanks of the inner thread that face the shavings, abrasion or removal of material can occur and lead to defects in the inner thread. Such defects can, in turn, impair the seating behavior of a screw element that is screwed into the inner thread.
Known from DE 38 80 394 T2 is a combined tool bit for drilling a hole and for cutting a thread. First of all, a core-hole bore is produced using the tap drill bit. Subsequently, the tap drill bit is moved with its tool bit axis in a circular path around the drilling axis and, in fact, this is conducted with rotation of the tap drill bit, as a result of which the thread profile produces an inner thread in the core-hole bore. Essentially the same method is also known from DE 39 39 795 C2 and from U.S. Pat. No. 5,678,962.